A method for producing a conducting metal-containing diamond-like nanocomposite film containing, as basic elements, carbon, silicon, metal, oxygen and hydrogen, and disposed on a dielectric substrate is known from U.S. Pat. No. 5,352,493. The method for producing the film is the following: A holder with a substrate of a dielectric material is placed in a vacuum chamber. A voltage of 0.3-5.0 kV with a frequency within the range of 1-25 MHz is applied to the substrate holder, and the substrate is maintained at a temperature of no more than 200-500° C. A gas discharge plasma with an energy density over 5 kilowatt-hour/gram-atom of carbon particles is generated in the vacuum chamber. Organosiloxane heated to a temperature of 500-800° C. is evaporated into the plasma to serve, while being decomposed in the plasma, as a source of carbon, silicon, oxygen and hydrogen. A beam of particles of a dopant in the form of atoms or ions is introduced into the vacuum chamber along with the plasma. A pressure of no more than 3×10−4 torr is maintained in the vacuum chamber, and atoms or ions of carbon, silicon, oxygen and hydrogen, as well as atoms or ions of the dopant are deposited on the substrate.
The resulting film is an amorphous isotropic structure consisting of three networks: a basic network diamond-like carbon network where the atomic concentration of carbon in the film is 40% of the total atomic concentration of the elements; a second network of an oxygen-stabilized silicon network, effectively glassy silicon; a third network of metal atoms filling the nanopores in the system of the two other networks, where the atomic concentration of the metal atoms is 20-25% of the total atomic concentration of elements. The third network determines the conductivity of the film. Since the film is formed at low temperatures of 200-500° C., all three networks: the carbon network, the amorphous silicon network and the amorphous metal network, are coupled to each other by weak chemical links, and the film microstructure is in a metastable state. It is claimed that the film exhibits a high degree of stability at elevated temperatures as high as 1250° C. This compares with crystalline diamond, which exhibits stability to heat up to 1100° C. It is claimed that the key to such properties lies in the absence of clusters and planar formations larger than 10 Å in the film structure, enabling the structure to stay in the metastable state at high temperatures. It is also claimed that clusters break the local symmetry in the structure and act as degradation centers.
This conventional method for producing a metal-doped diamond-like nanocomposite film has significant deficiencies. Contrary to the claims in U.S. Pat. No. 5,352,493, recent detailed investigations have revealed that the structure of the highly doped film produced by the above method exhibits nanoclusters of a size in the range of 30-500 Å, which are revealed by investigations with a scanning tunneling microscope. Apparently due to the film porosity, which appears in this case under high temperature conditions in the presence of oxygen atmosphere, an increased diffusion of oxygen into the film takes place, accompanied by carbon bum-out at high temperatures. Moreover, the clustering of the film results in instability of the metastable nanostructure with weak chemical links when the film is heated in use to higher temperatures. For the aforementioned reasons, instability of the film microstructure is already seen at a temperature of about 600° C., and this is accompanied by a temperature and time instability of electrical resistance of the film. The produced films exhibit a high electrical resistance exceeding the values mentioned in the description to the patent, apparently due barrier resistances at interfaces between the clusters.
Furthermore, the diamond-like properties of the film are related to atomic concentration of carbon at a level of 40% of the total atomic concentration of the elemental composition. The method for producing the film does not guarantee, due to its complexity, one-to-one reproduction of atomic concentrations of the elements contained in the film. Reduction in the carbon concentration leads to loss of the diamond-like properties and decreases the percent yield of films with diamond-like properties of the total quantity of the produced films.
The diamond-like films produced by the above method and having an atomic concentration of carbon of about 40% and an atomic concentration of metal of 20-25% exhibit insufficient plasticity, leading to destruction of the film-substrate system at high temperatures if a film having a thickness from fractions of a micron to a micron is produced on a thin substrate having a micron-sized thickness. Such service conditions of the film appear e.g., when the film is used as a resistive impedance heated by a current.
The aforementioned deficiencies of the conducting doped diamond-like nanocomposite films restrict the range of their application in practice.
The closest prior art is a method for producing a conducting doped diamond-like nanocomposite film containing, as basic elements, carbon, silicon, metal, oxygen and hydrogen, including the steps of: disposing a holder with a substrate of a dielectric material in a vacuum chamber; applying a voltage of 0.3-5.0 kV with a frequency within the range from 1 to 25 MHz to the substrate holder and maintaining a temperature of the substrate within the range from 200 to 500° C.; generating, in the vacuum chamber, a gas discharge plasma with an energy density of more than 5 kilowatt-hour/gram-atom of carbon particles; evaporating into the generated gas discharge plasma an organosiloxane compound heated to a temperature of 500-800° C. to serve as a source of carbon, silicon, oxygen and hydrogen while being decomposed in the plasma; introducing a beam of particles of a dopant in the form of atoms or ions into the vacuum chamber with the gas discharge plasma; depositing atoms or ions of carbon, silicon, oxygen and hydrogen, as well as atoms or ions of the dopant on the substrate to obtain a conducting doped carbon nanocomposite film as described in Russian patent No.2118206 (1998).
The method is aimed at production of films having a uniform composition throughout the volume. The result is attained by maintaining a pressure of no more than 1×10−4 torr in the vacuum chamber before initiating the process of depositing the film. The voltage is subsequently applied to the substrate holder, and a ceramic leak valve through which an organosiloxane compound is supplied into the vacuum chamber is heated to a temperature of 500-800° C., and argon is supplied into the chamber until the plasma reaches a steady-state combustion. After holding during 5-10 min to establish thereby a stationary deposition process, the substrate is isolated from the plasma flow, and the pressure of argon in the chamber is raised to 2×10−3 torr. Then the organosiloxane compound delivery is started and a source of dopant particles is enabled at the same time, and after expiration of 3-4 minutes the substrate is no longer isolated in order to provide growth of the film on the substrate. The dopants are not restricted to metals, but may also be nonmetals, chemical compounds, etc. Selection of the dopant is dictated by predetermined properties of the films to be produced.
A deficiency of the closest prior art method is that, like the preceding conventional method, the produced films consist of nanoclusters and exhibit a metastable microstructure with weak chemical links. As the result, instability of the film microstructure is already seen at a temperature of about 600° C., leading to a temperature and time instability of electrical resistance of the film at elevated temperatures, thereby impairing the film resistance at high temperatures and with thermal cycling.
The aforementioned deficiencies of the prior art method for producing doped diamond-like films restrict the range of practical application.